that geographers call "diffusion"-- the spread of a phenomenon over space and time. in 1986, which was before the government began collecting information on aids, there was a zero rate of reported infection across the country. in 1987, infection of the population by the hiv virus was reported only in the areas surrounding kenya's three major cities-- mombasa, nairobi and kisumu. but in 1990, the picture changed dramatically as rates of infection continued to rise in urban areas and also began climbing in rural areas. ouma: what's happening in the case of aids is that it tends to, um, diffuse, um, hierarchically-- large urban center, second largest urban center, like that. at the same time, you're also finding this contagious diffusion, which is really the spread from the source region-- be it nairobi or mombasa-- outward to the surrounding areas.